What is texas's age limit for buying a gun ?

Federal law requires you to be 21 to purchase pistols, 18 for long guns, in ANY state, and MOST states say you have to be this age to possess one without an adult accompanying you, as well.
Doesn't matter WHAT use you want it for.

It is unlawful to knowingly sell, rent, give or offer to sell, rent or give any firearm to a person under 18 years of age, without the written consent of his parent or guardian.

Though depending on where you reside:

A municipality may not adopt regulations relating to the transfer, private ownership, keeping, transportation, license or registration of firearms, ammunition, or firearms supplies.

To obtain a "license to carry" the following applies:

Application for a license to carry a handgun concealed is made to the Department of Public Safety. It shall specify if it is valid for a revolver or semi-auto pistol based on the proficiency certificate.
The applicant must submit: a completed application form; two recent color passport photographs; fingerprints; proof of age (at least 21); proof of residency in Texas; a handgun proficiency certificate from a qualified handgun instructor; an affidavit stating that applicant has read and understands the law concerning a license to carry and the laws on use of deadly force and that the applicant fulfills all eligibility requirements; and an authorization to access records. The Department shall issue a license to carry a concealed handgun to an applicant if the applicant meets all the eligibility requirements and submits all the application materials. Eligibility requirements include no record of felonies, certain misdemeanors, addictions, mental illness or delinquency in child support payments or tax payments. The Department must issue or deny the license within 60 days of receipt of the completed application. The Department must be notified within 30 days of a name or address change.

The best source of information is obtained by contacting a law enforcement agency with in your residing area.